Exothermic centres

Fig. 7.28. Calculated maximum over-pressure ratio (referred to the initial pressure) in the exothermic centre for stoichiometric methane, acetylene, ethylene and ethane with air at (a) 1 atm, (b) 1500 K. Initial radius of the centre is 1 mm. Geometry is cylindrical. From...

The DSC curve determined at 20 °C/min shows a very sharp exothermic peak centred at 970 °C and another slightly broader exotherm centred at 1228 °C. The first exotherm is followed immediately by an endotherm, which is much more marked at low heating rates. These events are irreversible and do not reappear in consecutive heating cycles of the same sample. The thermal analysis data repwrted by Kirchner et al. (2007a) and Ozao et al. (2001)... 

D.S.C. Pure iPPO, on cooling at lO C/min erfiibited an exotherm centred at 11.5 C, indicating crystallisation. Ihis exotherm point, reproducible to 1.0 C, was taken to be the optimum crystallisation temperature (1 ). Ohis interpretation has been discussed by Teilel baum et al (23) in their work on the crystallisation of natural rul )er, an3 l Beck et al ( ) cm the crystallisation of isotactic polypropylene. 

A critical factor is the boiling temperature of the blowing agent and its relationship to the temperature of the walls of the mould and of the reacting mixture. There should be sufficient exotherm to vaporise the blowing agent in the centre of the reacting material but the mould walls should be sufficiently cool to... 

A mixture erupted vigorously one horn after preparation [1], Interaction (not vigorous) of amines and halocarbons at ambient temperature had been recorded previously [2], The presence of 5 basic centres in the viscous amine would be expected to enhance exothermic effects. 

The weakness of the covalent bond in dilithium is understandable in terms of the low effective nuclear charge, which allows the 2s orbital to be very diffuse. The addition of an electron to the lithium atom is exothermic only to the extent of 59.8 kJ mol-1, which indicates the weakness of the attraction for the extra electron. By comparison, the exother-micity of electron attachment to the fluorine atom is 333 kJ mol-1. The diffuseness of the 2s orbital of lithium is indicated by the large bond length (267 pm) in the dilithium molecule. The metal exists in the form of a body-centred cubic lattice in which the radius of the lithium atoms is 152 pm again a very high value, indicative of the low cohesiveness of the metallic structure. The metallic lattice is preferred to the diatomic molecule as the more stable state of lithium. 

Other reactions of F" with esters as a result of attack at the carbonyl centre are not expected because of the absence of exothermic reaction channels. 

Comisarow (1977) has shown that, in the gas phase, methoxide ions react readily with methyl trifluoroacetate and methyl benzoate by an SN2 mechanism, while no reaction is observed as a result of nucleophilic displacement at the carbonyl centre. As for the case above, the SN2 reaction is highly exothermic, while the same is not true for the equivalent of reaction (64b). There is at present no satisfactory explanation of why (64a) apparently proceeds very slowly in the gas phase. 

One problem encountered in the field is the apparent irre-producibility of the results of different workers, even those in the same laboratory. This is particularly the case with molar mass distribution and steric triad composition. The explanation of these apparent inconsistencies comes with the realization that the mechanisms are eneidic and the polymer properties are primarily determined by independent active centres of different reactivities and stereospecificities whose relative proportions are set at the initiation step, which is completed in the first seconds of the polymerization. The irreproducibilities arise from irreproducibilities in the initiation step which had not been thought relevant. Ando, Chfljd and Nishioka (12) noted that these rapid exothermic reactions tend to rise very significantly above bath temperature (we have confirmed this effect) and allowed for this in considering the stereochemistry of the propagation reaction. However our results show that the influence on the initiation reactions can have a more far-reaching effect. 

When the Thiele modulus is large Cam is effectively zero and the maximum difference in temperature between the centre and exterior of the particle is (- AH)DeCAJke. Relative to the temperature outside the particle this maximum temperature difference is therefore 0. For exothermic reactions 0 is positive while for endothermic reactions it is negative. The curve in Fig. 3.6 for 0 = 0 represents isothermal conditions within the pellet. It is interesting to note that for a reaction in which -AH- 10 kJ/kmol, ke= lW/mK, De = 10 5m2/s and CAa> = 10 1 kmol/m3, the value of Tu - Tx is 100°C. In practice much lower values than this are observed but it does serve to show that serious errors may be introduced into calculations if conditions within the pellet are arbitrarily assumed to be isothermal. 

Figure 3.6 shows that, for exothermic reactions (0 > 0), the effectiveness factor may exceed unity. This is because the increase in rate caused by the temperature rise inside the particle more than compensates for the decrease in rate caused by the negative concentration gradient which effects a decrease in concentration towards the centre of the particle. A further point of interest is that, for reactions which are highly exothermic and at low value of the Thiele modulus, the value of tj is not uniquely defined by the Thiele modulus and the parameters 0 and e. The shape of... 

Finally, it is now clear that electron transfer between metal centres occurs readily when there are small differences in redox potential. Reactions which are considerably exothermic may be slower than expected due to difficulty in dissipating the energy released.1409,1421... 

On TiCl2 and TiCl3, H- and CH3- also bind to the titanium centre, in spite of the exothermicity of ligand abstraction. The formation ofHCl or CH3CI is weakly exothermic ... 

The addition of H- to TiHCl, much more exothermic than that to TiCl2, involves the reactivity of the H ligand. Since the formation of H2 is strongly exothermic, the H radical binds to the H ligand rather than to the Cl ligand as protons also do (see previous section) or to the titanium centre ... 

Chemical reactivity is influenced by solvation in different ways. As noted before, the solvent modulates the intrinsic characteristics of the reactants, which are related to polarization of its charge distribution. In addition, the interaction between solute and solvent molecules gives rise to a differential stabilization of reactants, products and transition states. The interaction of solvent molecules can affect both the equilibrium and kinetics of a chemical reaction, especially when there are large differences in the polarities of the reactants, transition state, or products. Classical examples that illustrate this solvent effect are the SN2 reaction, in which water molecules induce large changes in the kinetic and thermodynamic characteristics of the reaction, and the nucleophilic attack of an R-CT group on a carbonyl centre, which is very exothermic and occurs without an activation barrier in the gas phase but is clearly endothermic with a notable activation barrier in aqueous solution [76-79]. 

The course of these additions of lithium hydride resembles that found for the addition of borane (Nagase et al., 1980 Graham et al., 1981). With ethylene, the initial step is exothermic formation of a Jt-complex without barrier, then rate-determining transformation to the borane via a four-centre transition structure. In both the borane and lithium hydride additions, there is relatively little development of the new C—H bond with distances of 1.692 and 1.736 A respectively in the transition structures. When a carbanionic product is not formed, for example in the reaction of lithium hydride with cyclopropenyl cation yielding cyclopropene and lithium cation (Tapia et al., 1985), reaction again occurs via a hydride-bridged complex, but the C- H- -Li array remains nearly linear throughout the reaction. 

Endothermic occlusion takes place by diffusion of hydrogen into a metal lattice which is very little changed by the process. In exothermic occlusion by palladium, however, the face-centred cubic lattice (a phase) of palladium, with lattice constant 3.88 A, will accomodate, below 100°C, no more than about 5 at. % hydrogen, and then undergoes a transition to an expanded phase ( 3 phase), with lattice constant 4.02 A and H/Pd = 0.5—0.6. The H—Pd system thus splits into a and 3 phases in the manner familiar for two partially miscible liquids. The consolute temperature (rarely observable for solid phases) is about 310°C at H/Pd = 0.22. The phase diagram is, however, not well established because formation of the... 

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